Apparatuses consistent with the present invention relate to over-the-air broadcasting of digital television (DTV) signals, and more particularly, to receivers for such broadcast DTV signals.
The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as “A/53” for sake of brevity Digital television. A/53 specifies the eight-level-modulation vestigial-sideband amplitude-modulation signals known as “8VSB” signals that are used for over-the-air DTV broadcasting in the United States of America. In late May 2009 ATSC completed the writing of a “Candidate Standard: ATSC Mobile DTV Standard”, referred to hereinafter simply as “A/153” for sake of brevity, which candidate standard is incorporated herein by reference.
A/153 is directed to transmitting ancillary signals in time division multiplex with 8VSB DTV signals, which mobile/handheld (M/H)-service signals are designed for reception by mobile receivers and by hand-held receivers. The M/H-service data referred to as “M/H-service data” employ internet protocol (IP) transport streams. The M/H-service data are randomized and subjected to transverse Reed-Solomon (TRS) forward-error-correction (FEC) coding before serially concatenated convolutional coding (SCCC). The SCCC incorporates the 12-phase ⅔ trellis coding of 8VSB as inner convolutional coding following single-phase outer convolutional coding and intermediate symbol-interleaving procedures. The symbol-interleaved outer convolutional coding is time-division multiplexed into 8VSB DTV signal so as not to be subject to the convolutional byte interleaving prescribed by Section 4.2.5 of Annex D of A/53 and applied to the main DTV signal.
The 8-level symbol mapping specified in A/53 and in A/153 maps each group of Z2, Z1 and Z0 bits into a respective eight-level VSB symbol in accordance with simple binary coding. This results in the Z2 and Z2 bits of original data both changing value between the 011 and 100 levels. This makes a double-bit error likely when noise causes an adjacent-bin error during data slicing in this region of the symbol map. Only Reed-Solomon coding with 8-bit bytes is concatenated after the ⅔ trellis coding in ordinary 8VSB transmissions as specified by A/53, so the double-bit errors being within single bytes affect overall coding being found correct no more than single-bit errors within single bytes. However, when further convolutional coding is introduced before the ⅔ trellis coding at the transmitter, the double-bit errors are more disruptive than single-bit errors when decoding that further convolutional coding in the receiver.
Digital transmission systems using multi-level symbols generated by Gray coding are known. An adjacent-bin error will cause only a single-bit error in an 8-level symbol using Gray code symbol mapping, rather than a double-bit or triple-bit error. However, symbol mapping using Gray code over all eight modulation levels is incompatible with the ⅔ trellis coding of ordinary 8VSB coding. The ⅔ trellis coding must be maintained so as not to disrupt the operations of receivers already in the field that were designed for receiving ordinary 8VSB signals broadcast per A/53. So, initially, the inventor was unable to discern how to utilize effectively the general idea of avoiding an adjacent-bin error during data slicing generating double-bit errors in a special type of turbo coding designed for digital television broadcasting.
After further consideration, the inventor was able to figure out how to avoid generating double-bit errors in symbols each composed of a Z-sub-2 bit and a Z-sub-1 bit, which errors arise from adjacent-bin errors during data slicing. Each 2-bit symbol composed of a Z-sub-2 bit and a Z-sub-1 bit could be anti-Gray coded before ⅔ trellis coding in the DTV transmitter. Then, subsequent to ⅔ trellis decoding in the DTV receiver, each 2-bit symbol could be Gray coded to counter the effects of the anti-Gray coding. The symbol mapping into modulation levels is converted to Gray coding insofar as the two more significant bits of the 3-bit symbols are concerned. This procedure extends the effects of the ⅔ trellis decoding from just the Z-sub-1 bits to the Z-sub-2 bits as well, in a unique way quite different from the prior art.
U.S. Pat. No. 5,825,832 issued 20 Oct. 1998 to V. Benedetto and titled “Method and device for the reception of signals affected by inter-symbol interface” describes decoding procedures for serially concatenated convolutional coding (SCCC) that use cascaded Viterbi decoders, but do not employ a turbo decoding loop. A first Viterbi decoder supplies hard decisions as to the transmitted symbols accompanied by a reliability parameter. This soft-decision output from the first Viterbi decoder, which is essentially intended to take into account the memory effects of the channel by counteracting the effects of inter-symbol interference, is fed after de-interleaving to a second Viterbi decoder which carries out the actual decision. This decoding operation corresponds to the open-loop operation of a turbo decoding loop for SCCC. Interestingly, using single-dimension symbol mapping defined according to consecutive binary numbers will aid the first Viterbi decoder in its task of counteracting the effects of inter-symbol interference, since there are more transition points in the coded bits than there are using single-dimension symbol mapping defined according to Gray coding. The conversion of the symbol mapping after the first Viterbi decoder, so the second Viterbi decoder is presented with single-dimension symbol mapping defined according to Gray coding will benefit the second Viterbi decoder making actual decisions. This is because there are fewer transition points in the coded bits to affect decisions than with single-dimension symbol mapping defined according to consecutive binary numbers. Accordingly, there is apt to be a reduction in the number of decoding iterations required when turbo decoding procedures are implemented. Possibly, there will be some reduction in the SNR required to achieve satisfactory reception.
No matter what type of symbol mapping is used, the ⅔ trellis coding provides information for resolving adjacent-bin errors. When the corruption of the 8VSB symbols by noise is not severe, a DTV receiver will be able to decode the ⅔ trellis coding and correct adjacent-bin errors in the coded symbols, whether the errors be double-bit or single-bit in nature. When a spike of noise energy or a drop-out in received signal obliterates a few 8VSB symbols or causes distant-bin errors in the coded symbols, the information for resolving adjacent-bin errors in subsequent symbols is corrupted and is apt to generate recurring error for some time. The code pattern will probably eventually be such that the error would self correct. Similar effects occur for the convolutional outer coding.
In some DTV receiver designs, in order to shorten the time to recover from a spike of noise energy or a drop-out in received signal, the results of data-slicing 8VSB symbols are used to start the ⅔ trellis decoding procedure over. Gray coding the hard-decision portions of the results of data-slicing 8VSB symbols, as expressed in the soft decisions from the decoder for the inner convolutional coding, benefits the decoder for the outer convolutional coding. This is because the probability of error in the least significant bit of the symbol extracted from data slicing is reduced by at least one third.
There has been considerable development work done in DTV receiver design that incorporates the Viterbi decoder for the ⅔ trellis coding of ordinary 8VSB coding into the adaptive channel equalization filtering used to counteract the effects of inter-symbol interference. Deferring single-dimension symbol mapping being defined according to Gray coding until after both adaptive channel equalization and the Viterbi decoding procedure used to implement the adaptive channel equalization preserves the benefits of that previous development work.
After the inventor's insight into how to avoid an adjacent-bin error during data slicing generating double-bit errors in symbols each composed of a Z-sub-2 bit and a Z-sub-1 bit, there remained further problems of designing DTV transmitter and DTV receiver configurations to exploit the insight. U.S. patent application Ser. No. 11/978,462 titled “System for digital television broadcasting using modified ⅔ trellis coding” and filed by A. L. R. Limberg on 29 Oct. 2007 was published 15 May 2008 with publication No. US-2008-0112502-A1. The DTV receiver designs described in application Ser. No. 11/978,462 re-code soft decisions from the ⅔ trellis decoder to generate interleaved outer coding for subsequent de-interleaving and decoding. This re-coding is described as being performed by read-only memory (ROM) addressed by two-bit symbols. The inventor subsequently found that this symbol re-coding is better performed by ROM addressed by each successive complete soft decision supplied by the ⅔ trellis decoder. The DTV receiver designs described in application Ser. No. 11/978,462 re-code soft decisions from the outer SISO decoder for subsequent derivation of extrinsic information fed back to the ⅔ trellis decoder to implement turbo decoding. This symbol re-coding is also described as being performed by read-only memory addressed by two-bit symbols. The inventor subsequently found that this symbol re-coding also is better performed by ROM addressed by each successive complete soft decision supplied by the outer SISO decoder. That is, the ROMs used for symbol re-coding need to be addressed by the several bits descriptive of the two soft bits in each soft decision they are to re-code.
The ROMs used for symbol re-coding become quite large when addressed by the several bits descriptive of the two soft bits in each soft decision they are to re-code. E.g., addressing can be sixteen bits wide. However, the inventor has subsequently discerned that simple logic circuitry can be used for re-coding symbols from a mapping for binary-code modulation to a mapping for reflected-binary-code modulation—i.e., for Gray-code modulation—or vice versa. Symbol re-coders using such simple logic circuitry are used in receivers that embody the invention in its preferred forms.